Directional antenna signal combining arrangement and phase shifters therefor



1969 R. B. EN-EMARK ETAL 3478'269 DIRECTIONAL ANTENNA SIGNAL COMBININGARRANGEMENT AND PHASE SHIFTERS THEREFOR Filed Nov. 4. 1964 20! f g cf 5i #9 N a, g :1 711T Z! 1-"- IV? 1 Q: 1 E

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United States Patent ABSTRACT OF THE DISCLOSURE Signals from directionalantennas are combined by phase shifting one signal an anglecorresponding to the spatial agle of the antennas, and summing it withthe other signal. The phase shift network produces, through specifictransfer functions, optimalized system performance. It is formed with abalanced symmetrical lattice network using only passive impedanceelements, and terminated by a resistance, or with an unbalanced networkemploying amplifiers.

The field of the present invention is that of receiving systems forelectromagnetic waves wherein several wave collecting means are combinedfor obtaining for final detection a signal of predetermined directionalpattern.

It is well known to combine electromagnetic wave signals collected byseveral antennas of similar collecting pattern, such as loop antennas,for the purpose of obtaining predetermined patterns. If sucharrangements depend on phase shifting circuitry, such circuitry has tofulfill requirements such as to precision and range dictated by theperformance specifications for the system as a whole, and this presentsproblems that require particular consideration and for which solutionsare not readily available.

Objects of the invention are to provide a radio receiving system havingin effect an optimally omni-directional antenna pattern derived fromseveral antennas having directional radiation patterns:

To provide such a system which includes directional antennas that arenot detrimentally affected by operation within media, such as water,that attenuate the electric field component, and operates in such mediaas if it were fed by omni-directional, such as whip antennas that arewith omni-directional response, such as the response of a whip antenna,but with responsiveness principally dependent upon the relativelyunattenuated magnetic component of electromagnetic waves;

To provide such an omni-directional receiving system which includesantenna structures that are less susceptible to electrostaticinterference, such as precipitation static, than omni-directionalantenna structures;

To provide such a system which is especially suitable for use inconjunction with a pair of crossed loop antennas, which provides anoptimally uniform omni-directional reception pattern, which is availablefor either the tuned or broad band modes of reception, which has anormally satisfactory operating range over carrier frequencies differentby a ratio of about 2.5 while maintaining good circularity of theomni-directional receiving pattern to within 1 to 1.5 decibels, andwhich uses only linear circuit elements so that distortion of thereceiving pattern does not develop from interfering signals outside thecarrier frequency band of interest; and

To provide such a system which is comparatively simple, employs onlyeasily obtainable standard components, is rugged, and simple to set up,calibrate and operate.

The substance of the invention can be briefly summarized as involving ina broad aspect the reception of an electromagnetic wave signal, comingfrom a given direction, by two directional antennas effectivelydisplaced at an angle to each other, by shifting the phase of the twosignals an angle corresponding as closely as possible to the angulardisplacement of the two antennas and by then summing the thus phaseshifted signals in such a manner that the resulting output signal isvirtually undistinguishable from a single signal of optimalomni-directional characteristics. In a presently most importantpractical aspect, the directional antennas are a pair of loop antennascrossed at right angles and the phase of one of the two antenna signalsis accordingly shifted for combination with the other signal ofunmodified phase.

In another important phase, the system according to the inventionincludes a phase shift network which is particularly suitable forpurposes of the receiving system according to the invention as a whole.In its general aspect, this phase shift network is described by aphysically realizable transfer function including terms which permit thederivation of practical network component ratings producingpredeterminable and exactly prescribed tolerances for optimalized systemperformance.

In one specific practical aspect the phase shift network is realized asa balanced symmetrical lattice network using only passive impedanceelements, with two equal series arms (each having a capacitance in shuntwith a series combination of resistance and capacitance), with two equalshunt arms (each having an inductance in series with a parallelcombination of resistance and inductance), and terminated by aresistance.

In another specific practical aspect the phase shift network is realizedas an unbalanced network having a first arm with a series resistor and aresistor and an inductor parallel thereto, feeding into a firstamplifier which feeds into a second arm having two parallel branches onewith a resistance and the other with an inductance, the branches eachbeing earthed through a resistor; the potential difference developedacross the resistances is amplified in a differential amplifier whichhas output reference to earth; the amplifiers have high inputresistance, low output resistance, and low phase shift.

These and other objects and aspects of novelty of the invention willappear from the following description of its principle and mode ofoperation, and of several embodiments illustrating its novelcharacteristics.

The description refers to a drawing in which FIG. 1 is a block circuitdiagram of the receiving system according to the invention;

FIG. 2 is a plot of the transfer function of the phase shift networkincorporated in FIG. 1;

FIG. 3 is the block diagram of a practical phase shift network which isa component of FIG. 1;

FIGS. 4 and 5 are circuit diagrams of the arms of the network accordingto FIG. 3; and

FIG. 6 is the circuit diagram of another practical phase shift networksuitable as a component of FIG. 1.

In FIG. 1, L1 and L2 are two loop antennas of conventional design whichmay be crossed at 90 or which may be separately mounted at a similarangle. These antennas are connected to amplifiers A1 and A2 which areconventional variable gain control amplifiers permitting the adjustmentof the signals from L1 and L2 to equal amplitudes. The output ofamplifier A1 is fed into a high quality phase shift network P which willbe described more in detail hereinbelow. The output signal from thephase shift network P is fed to an amplifier A3 of conventional type,capable of adjustment to compensate for losses through the phase networkand thus permitting derivation of an optimally omni-directionalreception pattern. The phase shifted signal amplified at A3, and thesignal from loop L2, amplified at A2 as mentioned above,

are combined in a summing amplifier of conventional design indicated atA4, which provides the composite omnidirectional output signal. Such anamplifier is for example described under adding Network on page 458 ofReference Data for Radio Engineers, International Telephone andTelegraph Coroporation, fourth edition, 1956. Only two inputs of thisnetwork, supplied from A2 and A3, respectively, will be used.

The composite signal A4 is fed to a cathode follower amplifier A5 ofconventional design providing a low driving source impedance. Suitableamplifiers of this type are for example described on pages 448 to 450 ofthe above I'IT reference Data Handbook.

All amplifiers used in this circuitry are preferably vacuum tube deviceswhich, it was found, provides for greater flexibility in circuit designand performance. It is however understood that transistorized circuitryof similar performance characteristics can be used if carefully designedfor the purpose at hand.

It will be evident that, as indicated above, the quality of thereceiving system as a whole depends on phase and amplitude linearity ofthe phase shift network P which should maintain a 90 phase shift toWithin a few degrees, and a negligibly small gain change, between itsinput and output signals over a wide band such as 3 octaves centeredabout the chosen optimal carrier frequency. Referring to FIGS. 2 to 6,the practical construction of two precision phase shift networks P,fulfilling these conditions, will now be described. This description isbased on the following theoretical considerations.

A constant amplitude transfer function can be written according toconventional circuit theory (compare for example E. A. Guillemin,Synthesis of Passive Networks, Wiley, 1957, page 194) for a terminated,symmetrical, constant resistance lattice network as follows:

12 a b) a b) For an all-pass transfer function with poles and zerossymmetrical about the j-axis, /Z /Z is a reactance function. With Z Z=l, S=jw, and Z =jX we then have wherein 5:2 tan X, is the transferfunction phase with phase slope dfi/dw=(Sin [3)/w. Since X is a purereactance it is a function of m with positive slope so that dfl/dw 0.Thus, in order to qualify as an all-pass network, the ideal phaseshifter must have a phase characteristic satisfying dfl/dwzsin fi/w, andd 8/dw 0.

Assuming now, that, as in the present specific embodiment the desiredphase shift is approximately 1r/ 2, then dfi/dw l/w and Ar3=ln (to /mFor a frequency range as for example such that w =2w the numerical valueof AB would thus be 0.7 radian. This is much greater than can betolerated in a phasing network useful in the present system. However, ifinstead of an all-pass transfer function a network is accepted whichmerely approximates the ideal characteristic with a permissible amountof deviation over the entire frequency range, a practical solution canbe found. For that purpose, conditions for a permissible amount ofdeviation must be established, for example as follows.

With v =v +v wherein v is the output voltage of loop L1 and v that ofloop L2 through the phase shifter, it can be shown that wherein A isamplitude, 0 the phase shift, and for purposes of simplification,

The above formula for v indicates that, if a practical omni-directionalpattern with an ellipticity of less than :1 db is admissible, therequirements of (AA/A) 0.l and 0.1 radian must be fulfilled.

In order to accommodate the above conditions leading to a phase shiftnetwork that is practical for present purposes of an omni-directionalreceiving system, the restriction of using an all-pass transfer functionis abandoned; instead a more general algorithm is now adapoted which isindicated by Guillemin,'supra, at page 316 et seq., at follows.

With P(7\) a rational polynominal function of x, and with Q( anotherrational polynominal function of A, it is possible to Write Z =P()\)/Q(Therefore, the phase function must be specified in such a way that itstangent is a rational function of polynominals, and then P( \)Q()t)=M+Nand the zeros of M+N can be assigned to P()\) and Q( to form thetransfer function.

As a new concept in this algorithm, it can now be specifield that tanB=a(w/w )/[(w/w 1]. The constant a must be sufiiciently large so thatthe phase deviates from rr/Z by no more than the permissible amount forthe operating frequency range. For example, if we require that and0.45w/w g25, then the constant a must be greater than 21.

Quite generally, the transfer function can be written as where G is thegain of the phase shift network, for example the ratio v /v of theoutput and input voltages, or i where G'=[G|e with and where and 4: arethe input and output phase angles, respectively. In this transferfunction there is further X (S/S where S =-w '=(211'f) with f thefrequency of the input sinusoidal wave form, in cycles per second.Further, S is a constant which depends on the chosen carrier frequency fat which optimalized performance is desired; in general, f =S 211-. Iffd= /21r c.p.s., then S =1, further \=S, and still further a =a =a =a =LThis optimalizing procedure corresponds to conventional so-calledfrequency scaling techniques, compare for example E. A. Guillemin,Introductory Circuit Theory, Wiley, 1963, pages 309 ff.

With the above-mentioned newly introduced constant a chosen arbitrarilyas a=25, we have, with the above substitutions, N=25S and M=S +1 whichlatter it will be noted appears in the general transfer function G.There is further The assignment of zeros to P and Q is guided by therestriction that the degree of P must not exceed that of Q by unity.With P(S)=1 and Q( (s+0.04 (SH-24.96) then Z =1/[(S+0.004) (SA-24.96)].

It can be shown that |1z is far from unity for 0.4 w 2j. Therefore, somecompensation for amplitude gain as well as for phase shift is required.

For purposes of amplitude compensation, we can assign a pair of complexpoles or zeros, or a pair of real poles or zeros symmetrical about thej-axis. Introduction of these poles and zeros will not modify the phaseshifting operation in any way.

Further for purposes of amplitude compensation, to obtain a nearlyconstant response in the frequency range of interest, there can be putin the numerator of the transfer function the factor (S 1); a numeratorwas introduced into the transfer function G above for this very purpose.The transfer function thus becomes This function is plotted in FIG. 2where performance has been optimalized for carrier frequencies near /21rc.p.s.

For purposes of a practical embodiment, the last mentioned transferfunction can be realized in a physical, balanced, symmetrical latticenetwork, either directly from this formula or from the plot FIG. 2. Sucha network, PI, is shown in FIGS. 3 to 5. The component impedances are Z=(1-Z )/(l+Z and Z Z =L Using the above formula for Z the lattice armvalues are obtained as FIG. 3 shows the symmetrical lattice arms Z,, andZ of the network PI and FIGS. 4 and 5 indicate the ratings of theindividual elements of the two blocks Z and 2 As a second practicalembodiment of the phase shifter P, an unbalanced network PII (FIG. 6)will now be described.

An unbalanced phase shifting network is sometimes desirable, but thecircuitry according to FIGS. 3 to 5 cannot be rendered unbalancedwithout nearly ideal transformers. If available transformers are notsufficient for that purpose, if active circuits are permissible, and ifconstant input impedance is not perticuiarly important, a network suchas PII according to FIG. 6 is quite satisfactory.

1116 ratings of the passive elements are written into MG. 6. The activeelements consist of conventional amplifiers A6 and A7. The amplifier A6can for example be a grounded cathode-grounded plate pair as describedin the above referred to ITT Reference Data on page 446. The amplifierA7 can for example be a dilferential amplifier as described on page 447of the same Reference Data book. These amplifiers are required to haveeffectively infinite input and effectively zero output impedances. Inthe example according to FIG. 6, the input impedance is Z =24.96 -|-Sohms, and the gain with S=jw as referred to above. In this context itwill be remembered that for arithmetical simplicity, the phase shiftnetwork has been optimalized for carrier frequencies near /211- c.p.s.

The above described broad band omni-directional radio receiving systemhas in actual performance been optimalized for an operating range from14 to 35 kc. producing circularity to within one to one and one-halfdecibels, and beyond that range still useful operation to about 65 kc.without serious pattern distortion. The above described phase shiftnetwork which is of primary importance in systems of this type providesphase and amplitude linearity of the 90 phase shift within a few degreesover a 3 octave band if contained in the above described system withdirectional loop antennas crossed at 90.

It should be understood that the present disclosure is for the purposeof illustration only and that this invention includes all modificationsand equivalents which fall within the scope of the appended claims.

6 We claim: -1. An essentially omni-directional radio receiving systemcomprising:

tWo loop antennas placed at right angles;

means for shifting the phase of the signal from one of said antennas atright angles; and

means for summing the signal from the other antenna and the phaseshifted signal to furnish a single signal which essentially conforms toa single unidirectional system,

said phase shifting means comprising a balanced symmetrical latticenetwork having:

two equal impedance series arms each having a capacitor in shunt with aresistor and a capacitor in series;

two equal impedance shunt arms each having an inductor in series with aresistor and an inductor in parallel; and

a resistor terminating the network.

2. An essentially omni-directional radio receiving system comprising:

two loop antennas placed at right angles;

means for shifting the phase of the signal from one of said antennas atright angles; and

means for summing the signal from the other antenna and the phaseshifted signal to furnish a single signal which essentially conforms toa single unidirectional system,

said phase shifting means comprising an unbalanced network having:

a first arm having a series resistive branch, and a shunt branch havinga resistor and an inductor in series;

a first amplifier having its input connected to said first arm at thejunction of the branches of said first arm and having an inputresistance appreciably higher than the output resistance, and low phaseshift;

a second arm in series with said first amplifier and having a resistancebranch and an inductance branch, the output of the first amplifier beingconnected to the junction of said branches in the second arm;

earthed resistance means across said branches in said second arm; and

across said resistance means a differential amplifier having outputreference to ground, an input resistance appreciably higher than theoutput resistance, and low phase shift.

53. A phase shifting network in lattice formation comprrsmg:

' two equal impedance series arms each having a capacitor in shunt witha resistor and a capacitor in series; two equal impedance shunt armseach having an inductor in series with a resistor and an inductor inparallel; and a resistor terminating the network. 4. A phase shiftingnetwork comprising: a first arm having a series resistive branch, and ashunt branch having a resistor and an inductor in series; a firstamplifier having its input connected to said first farm at the junctionof the branches of said first arm and having an input resistanceappreciably higher than the output resistance, and low phase shift; a.second arm in series with said first amplifier and having a resistancebranch and an inductance branch, the output of the first amplifier beingconnected to the junction of said branches in the second arm; earthedresistance means across said branches in said second arm; and acrosssaid resistance means a differential amplifier havlng output referenceto ground, and input resistance appreciably higher than the outputresistance, and low phase shift. 5. A system for deriving an essentiallyomni-directional receiving pattern from two directional antennas placedsubstantially at a right angle in space, comprising:

balanced symmetrical lattice network means for relatively shifting thephase of the signals collected by said antennas an angle 3 approximatingsaid angle in space and related to the radian frequency w of saidsignals by the expression tan 6:

and two shunt arms of impedance where Z is given by the expression andwhere 7. A system according to claim wherein the constant a is selectedto be greater than about 20.

8. A system according to claim 7 wherein a is substantially equal to 25.

9. A system according to claim 5 wherein said phase shifting means isfurther characterized by a gain G given by the expression (a3)\) +a4\+a5 where x =(w/w the constants a1, a2, a3, a4, and a5 are selected sothat a4=a, and a1, a2, a3, and a5 are selected to furnish an acceptableamplitude deviation at a selected operating carrier frequency.

10. A phase shifting device in the form of a balanced lattice network,comprising means for shifting phase an angle 5 given by the expressiontan 5:

where w is the radian frequency of the input signal applied to saidnetwork, ca is the radian frequency of the signal carrier, and a is aconstant selected sufficiently large to produce a correspondingly smalldeviation in 5 over the desired operating frequency range.

11. A system according to claim 10 wherein said balanced symmetricallattice network has two series arms of impedance.

and two shunt arms of impedance 12 4 1-21.

where Z is given by the expression and where 2 2 EL (a) 12. A systemaccording to claim 10 wherein the constant a is selected to be greaterthan about 20.

13. A system according to claim 10 wherein a is substantially equal to25.

14. A system according to claim 10 wherein said phase shifting means isfurther characterized by a gain G given by the expression the constantsa1, a2, a3, a4, and a5 are selected so that (14:0, and a1, a2, a3, anda5 are selected to furnish an acceptable amplitude deviation at aselected operating carrier frequency.

References Cited UNITED STATES PATENTS 2,529,117 11/1950 Tompkins 333292,606,966 8/1952 Pawley 333-29 2,623,945 12/1952 Wigan 33329 2,951,1528/1960 Sichak et a1. 325-369 XR 2,982,924 5/1961 Weged et al 33329 XR3,242,430 3/1966 Chose 325369 XR ROBERT L. GRIFFIN, Primary Examiner R.S. BELL, Assistant Examiner US. Cl. X.R.

